SLOVYANSK, Ukraine (AP) -- Ukrainian forces launched an operation Thursday to drive pro-Russia insurgents out of occupied buildings in the country's tumultuous east, prompting new threats from Russian President Vladimir Putin.
Within hours of the Ukrainian operation, which killed at least two pro-Russia militants, Russia's defense minister announced new military exercises for troops massed near Ukraine's border.
The statements by Putin and Russian Defense Minister Sergei Shoigu sharpened anxiety over the prospect of a new Russian military incursion into Ukraine. Russia's foreign minister warned a day earlier that any attack on Russian citizens or interests in eastern Ukraine would bring a strong response.
In Kiev, Ukraine's acting president accused Russia of backing and supplying the separatists in the east and demanded that Moscow stop its intimidation campaign and leave his country alone.
Oleksandr Turchynov said in an address to the nation Thursday that Russia was "coordinating and openly supporting terrorist killers" in eastern Ukraine, where government buildings in at least 10 cities have been seized by pro-Russia gunmen.
Turchynov said Russia must pull back its troops from the Ukrainian border and "stop the constant threats and blackmail."
In St. Petersburg, Putin decried what he described as Ukraine's "punitive operation" and threatened Kiev with unspecified consequences.
"If the Kiev government is using the army against its own people this is clearly a grave crime," Putin said.
Russia already has tens of thousands of troops stationed in regions along its border with Ukraine. The latest Russian military exercises will involve ground troops in the south and the west and the air forces patrolling the border, Shoigu said.
NATO Deputy Secretary General Alexander Vershbow sharply criticized Russia for making "veiled threats" and said Russia should pull its troops back to their barracks.
The Ukrainian government and the West worry that Putin would welcome a pretext for a military intervention in eastern Ukraine. Putin denies that any Russian agents are operating in Ukraine, but insists he has the right to intervene to protect the ethnic Russians who make up a sizeable minority in eastern Ukraine.
Earlier in Tokyo, President Barack Obama accused Moscow of failing to live up to "the spirit or the letter" of a deal last week to ease tensions in eastern Ukraine. If that continues, Obama said, "there will be further consequences and we will ramp up further sanctions."
Pro-Russia forces say the interim government has also not lived up to the Geneva deal by not making nationalists in Kiev disarm and withdraw from their occupied buildings.
With no appetite in the U.S. for a military response, Obama is largely banking on Putin caving under a cascade of economic sanctions targeting his closest associates. But the success of that strategy also depends on European nations with closer financial ties to Moscow taking similar action, despite their concerns about a boomerang effect on their own economies.
"I understand that additional sanctions may not change Mr. Putin's calculus," Obama said while on a Tokyo visit. "How well they change his calculus in part depends on not only us applying sanctions but also the cooperation of other countries."
Slovyansk, a city 160 kilometers (100 miles) west of the Russian border, has emerged as the focus of the armed insurgency in eastern Ukraine.
The Ukrainian Interior Ministry said military and special police forces killed "up to five terrorists" while destroying three checkpoints north of Slovyansk on Thursday. One government member was wounded, it said.
Stella Khorosheva, a spokeswoman for the Slovyansk insurgents, said two pro-Russia fighters were killed at a checkpoint in the village of Khrestyshche, 10 kilometers (six miles) north of the city. She said checks were being made at hospitals to see if there were other casualties.
The situation was quiet in Slovyansk itself, but checkpoints inside the city were abandoned and it was unclear where the pro-Russia insurgents manning them had gone.
Khorosheva said the pro-Russia militia later regained control over the checkpoints where the clashes took place. By Thursday afternoon, an AP reporter confirmed that some of those checkpoints were back in the hands of insurgents.
Khorosheva declared that the fighters were ready to repel any attack by government troops.
"We will defend ourselves to our last drop of blood. We are ready to repeat Stalingrad," she told The Associated Press, invoking the memory of the Soviet army's victory over German forces in 1942-43.
At least 10 Ukrainian government armored vehicles were seen on the road north of Slovyansk and two helicopters circled over the area. Troops ordered residents in the area to keep away during the operation.
Near the town of Makatikha, several miles (kilometers) north of Slovyansk, pro-Russia militia set fire to rows of car tires in an apparent attempt to reduce the visibility from the air. An Associated Press reporter saw about two dozen militiamen manning checkpoints along the road earlier in the day.
In the southeastern city of Mariupol, Interior Minister Arsen Avakov said his forces had cleared city hall of the pro-Russia protesters who had been occupying it for more than a week. He did not describe the operation. The Ukrainian city sits along the main road between Russia and Russia's newly annexed Black Sea peninsula of Crimea.
Yulia Lasazan, a spokeswoman for Mariupol's police department, told The Associated Press about 30 masked men armed with baseball bats stormed the building in the dark early Thursday and started beating the pro-Russia protesters. Five people were taken to a hospital, she said.
It was not clear why the protesters, some of whom are believed to be armed, did not resist but called local police instead. Lasazan said local police now controlled the perimeter of Mariupol city hall and were negotiating to get the remaining protesters to leave.
Ukraine is going through its biggest political crisis since the 1991 fall of Soviet Union, set off by months of anti-government protests that led to President Viktor Yanukovych's flight to Russia in February.
Yanukovych's ouster sparked wide anger in his support base in Ukraine's east. The insurgents, who claim Ukraine's post-Yanukovych government consists of nationalists who will suppress the east, are demanding regional autonomy or even annexation by Russia.
Ukraine and Russia reached a deal in Geneva last week to defuse the crisis, but pro-Russian insurgents in the east - and right-wing militants in Kiev - have defied calls for all sides to disarm and to vacate the buildings they are occupying.